In some headers of a combine or forage harvester, crop is cut near the ground by an oscillating cutter. Though the cutter may use a scissoring action, it is commonly referred to as a “knife” and this term will be used herein to include any form of cutter. A reel with tines rotates in front of the knife about a horizontal axis to gather the cut crop and to drive it onto an auger which advances the crop towards the vehicle centerline. From there, conveyors carry the crop into the processing machinery. In the case of a combine harvester, the processing machinery separates the grain from the stalks and the chaff. The grain is stored in a grain tank while the stalk and the chaff are discharged from the rear of the harvester. In a forage harvester, the processing machinery chops the entire crop into small pieces and discharges it from a spout into a second vehicle driven alongside the harvester.
Commonly, the knife is mounted in a fixed position on the header and the various rotating and moving parts that drive the knife are covered by a guard. The guard is constructed as a door which can be hinged open to allow access to the parts which drive the knife. These parts include the so-called “wobble box,” which reciprocates a blade of the knife, and the pulley driving the wobble box.
In some headers, the knife can be moved forwards and backwards from a central position by about 50 cms. In headers having an adjustably mounted knife, a problem is encountered because the movement of the knife can result in some rotating and moving parts becoming exposed, as they are no longer shielded by the guard.